The present invention relates to a gear transmission, in particular for aircraft, featuring face gears, i.e. ring gears with face teeth, meshing with cylindrical pinions with axes perpendicular to the axes of the ring gears.
Face gears have the advantage of enabling the power input pinion to be defined by a straight or helical spur gear, so that meshing of the gears is unaffected by any axial movement of the pinion or, within reasonable limits, by displacement of the pinion in a direction parallel to the axis of the ring gear.
This enables the formation of transmissions in which the pinion meshes with and distributes torque to each of two coaxial ring gears, thus enabling compactness even in high-power applications with high transmission ratios.
To distribute the torque equally, transmissions are used, for example, in which the drive gear is defined by a floating pinion carried by the power input shaft, and which meshes with two coaxial counter-rotating ring gears facing each other and in turn meshing with one or more fixed transmission pinions rotating about respective axes substantially coplanar with the axis of the drive gear. The ability of the floating pinion to move in a direction parallel to the axis of the two ring gears enables the torque transmitted by the pinion to be distributed equally between the two ring gears regardless of inevitable dimensional tolerances of the gearing.
The above is a valid solution for transmissions with a single power input, as in single-engine helicopters.
In transmissions with more than one power input, however, as in multiple-engine helicopters, the need (for structural reasons) to provide more than one transmission pinion makes it difficult to distribute loads equally to transmission pinions with fixed axes of rotation. This balance depends on various factors involving the tooth thickness and position of the pinions; load dissymetry caused, for example, by nonsymmetrical drive pinions; and operating conditions with one engine excluded.